Insulin resistance is a classic feature of non-insulin dependent diabetes mellitus (NIDDM). Compelling evidence exists, from a variety of ethnic populations, to indicate that insulin resistance and hyperinsulinemia are early defects characterizing the prediabetic state. Thus, NIDDM features a primary, probably genetic, component of insulin resistance. However, the magnitude of insulin resistance is much greater in NIDDM than in prediabetic conditions. We hypothesize that this additional insulin resistance in NIDDM results from hyperglycemia through a form of glucotoxicity. Specifically we hypothesize that hyperglycemia causes impairment of insulin receptor tyrosine kinase activity though activation of serine and threonine kinase. This secondary component of insulin resistance is reversible, by correction of the hyperglycemia. A detailed elucidation of the cellular mechanisms underlying this phenomenon will open the possibility of new therapeutic approaches to NIDDM. We propose a combined in vivo and in vitro approach to test this hypothesis. We will induce in vivo hyperglycemia in various groups of human subjects at different insulin levels and conduct a thorough characterization of insulin action in vivo and in vitro before and after the hyperglycemia. We will assess the impact of hyperglycemia on insulin receptor signaling in adipocytes and skeletal muscle, in biopsies taken before and after hyperglycemia. We will test our hypothesis that diminished insulin receptor kinase activity results from increased serine/threonine kinase activity, probably through protein kinase C activation. We will first study lean control subjects but plan to extend the study to obese subjects, subjects with IGT and healthy elderly subjects, any or all of whom may be more susceptible to this secondary form of insulin resistance.